首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   2638篇
  免费   152篇
  国内免费   3篇
电工技术   31篇
化学工业   671篇
金属工艺   48篇
机械仪表   84篇
建筑科学   107篇
矿业工程   6篇
能源动力   98篇
轻工业   428篇
水利工程   23篇
石油天然气   10篇
无线电   165篇
一般工业技术   421篇
冶金工业   384篇
原子能技术   18篇
自动化技术   299篇
  2024年   7篇
  2023年   31篇
  2022年   117篇
  2021年   166篇
  2020年   82篇
  2019年   80篇
  2018年   103篇
  2017年   121篇
  2016年   101篇
  2015年   75篇
  2014年   110篇
  2013年   171篇
  2012年   170篇
  2011年   157篇
  2010年   133篇
  2009年   131篇
  2008年   135篇
  2007年   95篇
  2006年   96篇
  2005年   64篇
  2004年   59篇
  2003年   51篇
  2002年   51篇
  2001年   28篇
  2000年   18篇
  1999年   30篇
  1998年   52篇
  1997年   43篇
  1996年   46篇
  1995年   30篇
  1994年   27篇
  1993年   13篇
  1992年   17篇
  1991年   18篇
  1990年   24篇
  1989年   12篇
  1988年   11篇
  1987年   10篇
  1986年   11篇
  1985年   11篇
  1984年   12篇
  1983年   11篇
  1982年   7篇
  1981年   7篇
  1979年   4篇
  1978年   8篇
  1977年   6篇
  1976年   7篇
  1974年   4篇
  1970年   3篇
排序方式: 共有2793条查询结果,搜索用时 11 毫秒
61.
Estimation is not a purely scientific task. It requires cost engineers equipped with a wide knowledge of construction and a professional ability to make judgments. However, stress will deaden their use of knowledge and influence their professional judgment in estimating. In the construction industry, cost engineers can generally be divided into two main groups: clients’ cost engineers in consultant firms or in a development company who serve clients; and contractors’ cost engineers in construction companies working for the contractors. This study investigates the causal relationships between the stressors and stress of clients’ and contractors’ cost engineers. Two stressor–stress integrated models were developed for these two cost engineer groups by using the structural equation model. The findings revealed some stressors that affected both clients’ and contractors’ cost engineers: (1) poor environment, role conflict and work underload affect their stress positively; (2) social support and Type A behavior affect their stress negatively; and (3) work underload induces stress via the role conflict among cost engineers, their supervisors, and the organization. On the other hand, some deviations between these two groups of cost engineers also occur: (1) social support acts as a source of stressors of clients’ cost engineers; (2) a private life stressor only affects the stress of contractors’ cost engineers; and (3) a closed interactive looping relationship among role conflict, Type A behavior, and work underload affects contractors’ cost engineers. Based on the results of this study, some recommendations are suggested to manage cost engineers’ stress well.  相似文献   
62.
What unique assessment issues do evaluators face when conducting mental retardation evaluations in capital cases? This article describes 4 assessment issues and reports how 20 evaluators in 1 state have approached them: (a) What methods are appropriate for evaluating adaptive functioning in this context? (b) Should information about criminal behavior be used as evidence of adaptive functioning? (c) Should correctional officers serve as informants regarding adaptive behavior? and (d) How should the Flynn effect influence the interpretation of intelligence test scores? Implications for practice are that practitioners should expect to be asked about the relation between criminal behavior and adaptive functioning, must think carefully about the validity of tests of adaptive functioning for inmates, and must be knowledgeable about the current state of Flynn effect research. (PsycINFO Database Record (c) 2010 APA, all rights reserved)  相似文献   
63.
The results of 3679 patients, with intestinal obstruction, submitted to emergency surgery at the UUC-HCL between November 1981 and November 1991, were analysed in a general way, with the use of a graphic presentation. In the mechanical group, hernia (1604 cases), adhesions and bands (568 cases) and cancer (713 cases) were the most common pathologies; intestinal ischaemia (143 cases) was the most frequent form in the neurogenic group. Surgical therapy was evaluated in a general way. However, we comment on the evolution of primary surgical treatment of colorectal cancer in obstruction (625 cases). The mortality rate was in general: 10.8% (adults). In relative terms, the main features were intestinal ischaemia (39%), cancer (23%) and intestinal volvulus (22%).  相似文献   
64.
The present study measured how the detection thresholds for two very brief tone or noise bursts depend on the delay between them. The thresholds for the tone burst pairs systematically increase for increasing delays up to a few milliseconds and then reach a constant value. The tone burst thresholds thus reflect a very short-term interaction between the brief signals. Strikingly, the time constant of the interaction appears to scale inversely to the frequency of the tone bursts. The thresholds for the noise burst pairs, on the other hand, remain approximately constant down to the shortest measured delay of less than 1 ms. In contrast to the concept of intensity integration, these results are interpreted in terms of a temporal overlap of the auditory filter responses elicited by two brief, successively presented signals. In a second experiment it was shown that the observed short-term interaction between two brief signals is sensitive to their relative phases, corroborating the conjecture that the interaction takes place at a level prior to the mechanical to neural transduction in the inner ear.  相似文献   
65.
This paper describes the contribution of Fabry–Perot (FP) fiber optic temperature sensor to investigate the effects of concrete hydration process. The FP temperature sensor was easily fabricated by controllable chemical etching and adjustable fusion splicing. Detailed optical properties of the sensor were theoretically analyzed and temperature calibration experiments were performed. A sensor with a 90 μm cavity length was demonstrated to have a temperature sensitivity of 0.01 nm/°C and the linearity coefficient of 0.99. Furthermore, the FP sensor was embedded in the concrete structure for sensing the temperature change during the early age of hydration. During the concrete hydration experiments, the measured peak temperatures of the concrete specimens with different water-to-cement (w/c) ratios of 0.4, 0.5 and 0.6 were 51.42 °C, 52.88 °C, and 55.08 °C, respectively, corresponding to final setting times of 13.52 h (w/c = 0.4), 14.16 h (w/c = 0.5) and 15.2 h (w/c = 0.6) after concrete casting. Temperature profiles will be used for concrete hydration heat study, which will help us to have a better understanding of cement hydration behavior.  相似文献   
66.
The influence of multilayer composite surface coatings on gear scuffing load carrying capacity, gear friction coefficient and gearbox efficiency is discussed in this work.The deposition procedures of molybdenum disulphide/titanium (MoS2/Ti) and carbon/chromium (C/Cr) composite coatings are described.Tests reported in the literature, such as Rockwell indentations, ball cratering, pin-on-disc and reciprocating wear, confirm the excellent adhesion to the substrate and the tribological performance of these coatings, suggesting they can be applied with success in heavy loaded rolling–sliding contacts, such as those found in gears.FZG gear scuffing tests were performed in order to evaluate the coatings anti-scuffing performance, which both improved very significantly in comparison to uncoated gears. These results in conjunction with the friction power intensity (FPI) scuffing criterion allowed the determination of a friction coefficient factor XSC to include the coating influence on the friction coefficient expression.The composite coatings were also applied to the gears of a transfer gearbox and its efficiency was measured and compared at different input speeds and torques with the uncoated carburized steel gears. Significant efficiency improvement was found with the MoS2/Ti coating.  相似文献   
67.
Objective: Attentional control, the ability to maintain goal-directedness in the face of distraction, has been shown to decline in normal aging (NA) and Alzheimer's disease (AD), yet the nature and extent of deficits is under debate. This study investigated attentional control in NA and AD compared to healthy young adults in several tasks such as setting, suppressing, switching, and preparing attention. Method: Fifty-two participants (17 AD, 17 NA, and 18 young participants) underwent the Tower of London, the Zoo map test, the Stroop test, letter verbal fluency, a computerized version of the Rule shift cards tests, the Trail making test, the Plus-minus test, and a reaction time task with variable preparatory intervals. Results: Analyses of variance showed that NA as compared to young participants were impaired in the Tower of London, the Stroop test, and the Rule shift cards tests. AD as compared to NA participants were impaired in all tests except the Stroop test. Principal component analysis in young adults confirmed the modularity of attentional tasks, which was reduced in NA and AD participants. Principal component analysis in all populations showed a decline of attentional control with NA and AD regardless of the tasks, with an increase in between-participants variability only between young and NA participants. Conclusions: Attentional control dysfunction is different in NA and AD: NA affects suppressing attention, switching attention for unpredictable but not predictable events, and preparing attention for unpredictable events, whereas AD affects setting, suppressing, switching, and preparing attention with less specificity. (PsycINFO Database Record (c) 2011 APA, all rights reserved)  相似文献   
68.
Metabolic diseases, such as obesity, Type II diabetes and hepatic steatosis, are a significant public health concern affecting more than half a billion people worldwide. The prevalence of these diseases is constantly increasing in developed countries, affecting all age groups. The pathogenesis of metabolic diseases is complex and multifactorial. Inducer factors can either be genetic or linked to a sedentary lifestyle and/or consumption of high-fat and sugar diets. In 2002, a new concept of “environmental obesogens” emerged, suggesting that environmental chemicals could play an active role in the etiology of obesity. Bisphenol A (BPA), a xenoestrogen widely used in the plastic food packaging industry has been shown to affect many physiological functions and has been linked to reproductive, endocrine and metabolic disorders and cancer. Therefore, the widespread use of BPA during the last 30 years could have contributed to the increased incidence of metabolic diseases. BPA was banned in baby bottles in Canada in 2008 and in all food-oriented packaging in France from 1 January 2015. Since the BPA ban, substitutes with a similar structure and properties have been used by industrials even though their toxic potential is unknown. Bisphenol S has mainly replaced BPA in consumer products as reflected by the almost ubiquitous human exposure to this contaminant. This review focuses on the metabolic effects and targets of BPA and recent data, which suggest comparable effects of the structural analogs used as substitutes.  相似文献   
69.
The metabolic syndrome (MetS) is a cluster of cardiovascular risk factors characterised by central obesity, atherogenic dyslipidaemia, and changes in the circulating lipidome; the underlying mechanisms that lead to this lipid remodelling have only been partially elucidated. This study used an integrated “omics” approach (untargeted whole serum lipidomics, targeted proteomics, and lipoprotein lipidomics) to study lipoprotein remodelling and HDL composition in subjects with central obesity diagnosed with MetS (vs. controls). Compared with healthy subjects, MetS patients showed higher free fatty acids, diglycerides, phosphatidylcholines, and triglycerides, particularly those enriched in products of de novo lipogenesis. On the other hand, the “lysophosphatidylcholines to phosphatidylcholines” and “cholesteryl ester to free cholesterol” ratios were reduced, pointing to a lower activity of lecithin cholesterol acyltransferase (LCAT) in MetS; LCAT activity (directly measured and predicted by lipidomic ratios) was positively correlated with high-density lipoprotein cholesterol (HDL-C) and negatively correlated with body mass index (BMI) and insulin resistance. Moreover, many phosphatidylcholines and sphingomyelins were significantly lower in the HDL of MetS patients and strongly correlated with BMI and clinical metabolic parameters. These results suggest that MetS is associated with an impairment of phospholipid metabolism in HDL, partially led by LCAT, and associated with obesity and underlying insulin resistance. This study proposes a candidate strategy to use integrated “omics” approaches to gain mechanistic insights into lipoprotein remodelling, thus deepening the knowledge regarding the molecular basis of the association between MetS and atherosclerosis.  相似文献   
70.
Techniques used in different biological and industrial fields for the detection and quantification of tannin compounds in plants are discussed. Emphasis is placed on the choice of method for ecological investigations, which may benefit from the use of a method that measures relative astringency of plant extracts at reduced costs in time and equipment, permitting the analysis of larger samples than may be feasible with more sophisticated analytical techniques.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号